Electrostatic Chucks (ESCs) are essential to precision semiconductor wafer manufacturing process. Existing chucks may be divided into two major categories, each category having its particular strengths and weaknesses.
The dielectric in a Polyimide ESC (PESC) is a strong insulator, consequently, most of the applied voltage drop is across the dielectric and creates a Coulombic chucking force. Unfortunately, the PESC working surface is super-sensitive to scratching. Further, the PESC is susceptible particle embedding, which can cause arcing between the backside of the wafer and the copper electrode on the PESC. Still further, the PESC cannot be used at high temperatures because high temperature operation can cause blisters of water moisture through the polyimide layer.
The partially conducting Ceramic ESCs (CESC) require a constant current to achieve adequate chucking force, thus CESC leakage current is greater, requiring a larger power supply than a PESC. This current-dependent chucking force, known as the Johnsen-Rahbek Effect, is small compared to the Coulombic force in the PESC.
CESCs with anodized aluminum trioxide (Al2O3), currently available only as monopolar devices, are extremely sensitive to moisture. Further, anodized aluminum has been used as an ESC insulating layer in some bipolar ESCs. However, arcing and anodization defects often cause early failure of this type of ESC.
Doped alumina has been also used for some CESCs for etch applications. The doped ceramic helps to control its resistivity in Johnson-Rahbeck ESC resistivity range. But the grain boundary attack on glass phases can change the surface roughness of the ESC and therefore increase the resistivity. Further, the roughened ceramic surface will cause high helium leak. An attack on the ceramic surface during a wafer-less auto-clean cycle on ceramic grain boundaries often causes the impedance of the ceramic to move from a Johnson-Rahbeck type impedance to a Coulombic type impedance.
High purity ceramic, e.g., Alumina, has been widely used as the dielectric puck layer on a surface of ESCs. It has been used either as a monopolar or a bipolar ESC. Further, it is used as a Coulombic ESC due to its high resistivity. High purity alumina, e.g., 99.7% purity or higher, may be applied as ESC dielectric puck layer either using a solid sintering ceramic or using a thermal spray coating.
Recently introduced CESCs, employing a sintered Aluminum nitride (AlN) dielectric, have poor thermal transfer characteristics. Resistivity of the ceramic material is temperature-dependent and more variable from piece to piece than the PESC. AlN has a superior thermal conductivity in comparison with alumina. Therefore, it has received wide applications as a high temperature ESC operating at 200° C. or higher. In most cases, AlN surface has mesa surface patterns to control ESC contact area with a wafer surface. The major problem of AlN is that it can generate AlF3 particles when SF6, NF3 and other F-based gases are used in the etching chamber. AlF3 is one the major particle sources in etching chamber technology. Since AlN resistivity depends on the operating temperature as a Johnsen-Rahbek ESC, selection of suitable types of AlN to maintain a workable resistivity and maintaining high plasma-resistance under high density plasma are very important.
Depending on end user requirements and installed equipment, each of the PESC or CESC will satisfactorily hold (Chuck) and release (Dechuck). Generally speaking, it doesn't matter which type of ESC will be used, the capacitance and resistivity of an ESC are the two key parameters for ESC functionality.
FIG. 1 illustrates a planar view of a conventional bipolar electrostatic chuck (ESC) 100. ESC 100 has a top surface 102 and a mounting ledge 104. ESC 100 includes a first electrode 106 and a second electrode 108. First electrode 106 includes an inner electrode portion 110 and an outer electrode portion 112.
FIG. 2 illustrates a cross-sectional view of ESC 100 along line x-x. As illustrated in FIG. 2, ESC 100 includes a rear surface, or base, 114.
Mounting holes (not shown) on mounting ledge 104 enable mounting of ESC 100 onto a system.
In operation, a first voltage differential is applied across first electrode 106 and second electrode 108. The voltage differential creates an electric field, which is used to attract and hold a wafer for processing. When the processing is finished, a second voltage differential (dechucking voltage) is applied across first electrode 106 and second electrode 108 to release the wafer.
Although briefly described above, the voltage control on a conventional ESC, whether single or multi-poled is critical. In this light, many parameters of the ESC that may affect such voltage control are therefore also critical. Non-limiting parameters include resistance, capacitance, impedance and frequency phase shift. Further, the parameters may be further analyzed for each independent portion of the ESC, as opposed to analyzing the parameter of the chuck in its entirety. Non-limiting examples of which include, a specific parameter measured from one electrode to another electrode (pole-to-pole), a specific parameter measured from one electrode on the top surface to the base (pole-to-base).
FIG. 3 illustrates a conventional technique of measuring a parameter of ESC 100. Here, ESC 100 includes measuring terminals 312, 308 and 310, capable of permitting measurement of a characteristic of base 114, first electrode 106 and second electrode 108, respectively. Conventional measuring device 302 includes a first terminal 304 and a second terminal 306. In this example, conventional measuring device 302 may measure a characteristic of ESC 100 between two points. As illustrated, first terminal 304 may be either connected to measuring terminal 312 or measuring terminal 308, whereas second terminal 306 may be either connected to measuring terminal 312 or measuring terminal 310.
In this manner, when first terminal 304 is connected to measuring terminal 308 and when second terminal 306 is connected to measuring terminal 312, a characteristic of first electrode 106 may be measured using a pole-to-base measurement. Similarly, when first terminal 304 is connected to measuring terminal 308 and when second terminal 306 is connected to measuring terminal 310, a characteristic of first electrode 106 and of second electrode 108 may be measured using a pole-to-pole measurement. Similarly, when first terminal 304 is connected to measuring terminal 312 and when second terminal 306 is connected to measuring terminal 310, a characteristic of second electrode 108 may be measured using a pole-to-base measurement.
In the above discussed conventional technique, when conventional measuring device 302 is capable of measuring resistance, the user may measure any one of the resistance of first electrode 106 from pole-to-base, the resistance of second electrode 108 from pole-to-base and the resistance of first electrode 106 and second electrode 108 from pole to pole. Similarly, when conventional measuring device 302 is capable of measuring capacitance, the user may measure any one of the capacitance of first electrode 106 from pole-to-base, the capacitance of second electrode 108 from pole-to-base and the capacitance of first electrode 106 and second electrode 108 from pole to pole. When conventional measuring device 302 is capable of measuring inductance, the user may measure any one of the inductance of first electrode 106 from pole-to-base, the inductance of second electrode 108 from pole-to-base and the inductance of first electrode 106 and second electrode 108 from pole to pole. When conventional measuring device 302 is capable of measuring impedance, the user may measure any one of the impedance of first electrode 106 from pole-to-base, the impedance of second electrode 108 from pole-to-base and the impedance of first electrode 106 and second electrode 108 from pole to pole. When conventional measuring device 302 is capable of measuring a phase delay of the frequency of the applied voltage, the user may measure any one of the phase delay of first electrode 106 from pole-to-base, the phase delay of second electrode 108 from pole-to-base and the phase delay of first electrode 106 and second electrode 108 from pole to pole.
FIGS. 1 and 2 illustrate one type of conventional ESC, and in a very simplistic manner. Many features of the conventional bipolar ESC discussed above have not been shown or described to simplify the discussion. Further, many other types of conventional ESCs have not been specifically described to simplify the discussion. The important notion is that conventional techniques exist to measure specific properties of ESCs and individual portions thereof.
Reliable electrical performance is of the utmost importance. Accordingly, an ESC manufacturer may perform quality assurance checks on manufactured ESC before shipping to customers. One conventional quality assurance check may include determining whether a specific parameter of a manufactured ESC is within a predetermined acceptable range, non-limiting examples of which include: the measured resistance being equal to or greater than R1Ω and equal to or less than R2Ω; the measured capacitance being equal to or greater than c1F and equal to or less than c2F; the measured impedance being equal to or greater than Z1Ω and equal to or less than Z2Ω; and the measured frequency phase shift being equal to or greater than φ1 and equal to or less than φ2. In the conventional methods, the manufacturer applies a current or voltage to the terminals as discussed above at a predetermined frequency fm. If all significant characteristics, e.g., resistance as measured by an Ohmmeter, are within the manufacturer's predetermined range of acceptability, then the ESC is determined to be acceptable.
As an example in TABLE I below, resistance and capacitance measurements were taken on several ESCs (p/n 718-094523-281-E). Capacitance measurements ranged from 3.478 to 3.777 nano Farads and resistance measurements ranged from 2.267 to 3.829 Meg Ohms:
TABLE IDevice (Ser. No.)Capacitance (nF)Resistivity (Megohms)D194693.7263.829D173033.7773.244D184693.7253.829D174243.6402.672D176973.4782.267Note, these devices were deemed acceptable by the manufacturer. How close the first device tested was to exceeding the maximum allowable resistance, or the second was to having too little resistance hasn't been defined.
An ESC's performance will degrade with use. If visual inspection clearly indicates ESC defects (cracked, deep scratches, residual particles) immediate replacement may be in order. It is difficult to isolate the cause when an ESC fails to perform satisfactorily, because of the sensitivity and complexity of the process control system. The present state of the art does not provide a simple non-invasive means of detecting non-visual defects. Conventional characteristic tests, for example, duplicating the manufacturer's acceptance tests, as discussed above, may indicate that a particular ESC is acceptable even though the ESC does not perform satisfactorily.
What is needed is a reliable, non-invasive method to determine the suitability of an ESC for initial or for continued use.